In this specification, a person (or user) who is a target of heart rate measurement or heart rate estimation, will be referred to as a “subject”, for the sake of convenience. Various heart rate measuring apparatuses have been proposed to measure a heart rate by making contact with the subject. However, in the case of the contact type heart rate measuring apparatus, an exclusive sensor, such as an ear clip to be worn on the subject's ear, for example, is used. For this reason, the contact type heart rate measuring apparatus may not be easy to use, and the ear clip may come off from the subject's ear. Hence, it may be difficult to realize a heart rate measuring apparatus capable of positively measuring the heart rate while the subject moves, such as when the subject walks.
On the other hand, a non-contact type heart rate measuring apparatus may detect reflected wave from breast or the like of the subject irradiated with microwave, for example, and estimate the heart rate of the subject by utilizing the Doppler effect. More particularly, because the reflected wave from the subject changes when a distance between a microwave transmitter and the subject changes, the heart rate may be estimated by measuring a displacement of a body surface or an organ, such as the heart, caused by the heart beat. For example, the microwave may have a wavelength of 1 m to 100 pm and a frequency of 300 MHz to 3 THz.
However, the reflected wave from the subject may include reflected wave related to various information other than heart rate information. The reflected wave related to the various information other than the heart rate information may include the reflected wave from other than the body surface of the subject undergoing the displacement caused by the heart beat, such as the body surface of the subject undergoing a displacement caused by breathing, clothing worn by the subject, and the like. When the subject moves by walking, for example, the heart rate and a respiration rate tend to increase, and a shape of the clothing also changes. For this reason, the reflected wave related to the various information other than the heart rate information changes depending on a movement and the like of the subject.
The various information other than the heart rate information may be regarded as noise with respect to the heart rate information, and the accuracy of the heart rate estimation may be difficult to improve when the noise is mixed to the reflected wave. The accuracy of the heart rate estimation may be difficult to improve particularly because it is difficult to eliminate the noise that changes depending on the movement of the subject.
Various techniques are proposed in Japanese Laid-Open Patent Publications No. 2009-55997, No. 2011-50604, No. 2008-99849, No. 2005-237569, No. 5-288760, and No. 1-115344, for example.
In the conventional non-contact type heart rate measuring apparatuses, it may be difficult to improve the accuracy of the heart rate estimation because separation of the heart rate information, by eliminating the noise that changes depending on the movement of the subject, is difficult.